1. Field of the Invention
This invention relates to a rotary drum unit in a recording and/or reproducing apparatus such as a video tape recorder (hereinafter referred to as VTR).
2. Description of the Related Art
The structural arrangement of rotary drum units heretofore adapted for recording and/or reproducing apparatuses such as VTRs is, for example, as shown in FIG. 1. Referring to FIG. 1, a rotary drum unit 10 includes a rotary upper drum 11, a magnetic head 12 which is secured to the upper drum 11, and a stationary lower drum 13. A support shaft 14 is secured to the lower drum 13. A transformer base 15 for a rotary transformer 17 is rotatably supported by the support shaft 14 through a bearing 16. The rotary transformer 17 (rotor) is secured to the transformer base 15. A rotary transformer 18 (stator) is secured to the lower drum 13 in a state of being opposed to the other rotary transformer 17. A motor coil 19 is secured to the support shaft 14. A motor magnet 20 is secured to the upper drum 11.
FIG. 2 shows the rotary drum unit 10 in a state of operation, that is, recording or reproducing on or from a magnetic tape 21. The magnetic tape 21 is guided by a plurality of guide posts 22 to be smoothly and precisely wrapped around the upper drum 11 of the rotary drum unit 10.
With the rotary drum unit 10 arranged in this manner, when a predetermined current is allowed to flow to the motor coil 19, a rotating force is generated at the motor magnet 20. As a result, the upper drum 11, the magnetic head 12, the transformer base 15 and the rotary transformer 17 rotate. After that, the magnetic tape 21 comes to be obliquely wrapped around the rotary drum unit 10 as shown in FIG. 2 to make the VTR ready for recording or reproduction on or from the magnetic tape 21.
A plurality of grooves 23 which are for example in a triangular sectional shape are carved in the outer peripheral surface 11a of the upper drum 11 of the rotary drum 10, as shown in FIG. 3. These grooves 23 not only prevent the magnetic tape 21 from sticking to the outer peripheral surface 11a of the upper drum 11 when the rotary drum unit 10 is stopped, but also serve to minimize the thickness of a so-called air film which is generated over the outer peripheral surface 11a, so that the magnetic tape 21 can be allowed to travel in a stable manner.
The magnetic tape 21 comes into contact with the rotary drum unit 10 in the neighborhood of a point A shown in FIG. 2 on the tape entrance side. Then, if an excessively thick air film is generated, the contacting state of the magnetic tape 21 becomes unstable. The possibility of such unstable contact can be eliminated by forming the grooves 23 deep as shown in FIG. 4.
In the case of the conventional rotary drum unit 10, however, a difference between the thickness of an air film generated on the tape entrance side (the point A in FIG. 2) and the thickness of an air film generated on the tape exit side of the rotary drum 10 (a point B in FIG. 2) increases accordingly as the rotational frequency of the rotary drum 10 increases. The increase of the difference in air film thickness then brings about a difference in pressing contact force between the magnetic head 12 and the magnetic tape 21. The difference of the pressing contact force fluctuates the level of the reproduction output of the VTR. This tendency of fluctuations becomes more salient as the relative speed of the magnetic tape 21 and the rotary upper drum 11 is higher and also as the frequency of signals reproduced becomes higher.
Further, with the grooves 23 formed deep as mentioned above (FIG. 4), the thickness of an air film can be reduced to a certain extent on the tape entrance side. However, the deep grooves 23 cause the magnetic tape 21 to wave correspondingly with the grooves 23. The waving then causes the magnetic tape 21 not only to travel unstably but also to be damaged where the magnetic tape 21 comes into contact with the edge parts of the grooves 23 on the tape exit side if the grooves 23 are formed too deep.
This problem may be solved by increasing the number of the grooves 23. However, the maximum number of the grooves 23 to be formed is limited in relation to the depth of the grooves 23. It is, therefore, difficult in fact to simply increase the number of the grooves 23. Besides, the results of experiments have indicated that the advantageous effect attainable by increasing the number of the grooves 23 is limited. Further, some effect may be attained also by adequately setting some conditions including the rigidity of the magnetic tape 21, the traveling speed of the magnetic tape 21 relative to the rotary upper drum 11, etc. However, a desired effect of such solution is hardly expectable in cases where a relative speed between the upper drum 11 and the magnetic tape 21 is high and where the air film is thick.